1. Field of the Invention
The present invention relates to an electrode suitable for use with a silicon carbide semiconductor (hereinafter the electrode will be referred to as an xe2x80x9celectrode for a silicon carbide semiconductorxe2x80x9d), which can be reliably operated at high temperature; and more particularly to an ohmic electrode for a p-type silicon carbide semiconductor, a silicon carbide semiconductor element containing the electrode, a method for producing the electrode, and a method for producing the element.
2. Description of the Related Art
Silicon carbide (SiC) has a band gap as large as about 2.3 to 3 eV (3C-SiC has a band gap of 2.23 eV, 6H-SiC has a band gap of 3.03 eV, and 4H-SiC has a band gap of 3.26 eV; as used herein, the prefix letters C and H refer to xe2x80x9ccubicxe2x80x9d and xe2x80x9chexagonal,xe2x80x9d respectively, and the number 3, 4, or 6 associated with C or H refers to the number of repeating crystal structure units). Therefore, semiconductor properties of SiC are maintained even at a high temperature of about 600xc2x0 C., and applications of SiC to semiconductor elements which can be operated at high temperature are envisaged.
When a semiconductor element exhibiting heat resistance is produced from SiC, formation of an n-type or p-type stable ohmic electrode is an important issue. For example, Japanese Patent Application Laid-Open (kokai) No. 8-139051 discloses a method for forming an ohmic electrode by carbonizing a metallic layer formed on the surface of a substrate containing silicon carbide crystals, and subjecting the layer to heat treatment.
Journal of ELECTRONIC MATERIALS (Vol. 29, No. 3, 391-397, 2000) reports that when p-Si/Ta is deposited on a p-type SiC layer, and then heat treatment is performed in an H2-Ar gas atmosphere at 1,100xc2x0 C. for five minutes, Ta reacts with Si to form TaSi2, and p-type SiC containing a large amount of Si is generated in the vicinity of the SiC layer and the p-Si/Ta layer, resulting in formation of an ohmic electrode.
APPLIED PHYSICS LETTERS (Vol. 73, No. 14, 2009-2011, 1998) reports that when Si/Pt is deposited on a p-type 4H-SiC layer, and then heat treatment is performed at a temperature of 1,000xc2x0 C. or higher, an ohmic electrode is formed. This publication also reports that when heat treatment is performed at 1,100xc2x0 C., generation of a Ptxe2x80x94Sixe2x80x94C mixture layer through reaction between Pt and SiC is confirmed by means of Auger electron spectroscopy, and that the mixture layer has surface roughness.
When n-type SiC crystals (i.e., an n-type SiC semiconductor) are employed, an ohmic electrode having an ohmic junction can be formed by depositing on the n-type semiconductor a metallic compound having a work function lower than that of the n-type semiconductor. Examples of such a metallic compound include single metal elements, metallic alloys, and metallic compounds, such as Ni, Ti, Ta, W, NiSiX (nickel silicide), TaSiX (tantalum silicide), and WSiX (tungsten silicide); and mixtures thereof. These metallic compounds are known to have high melting points, to exhibit excellent heat resistance, and to be reliably employed at temperatures as high as about 600xc2x0 C.
Meanwhile, when p-type SiC crystals (i.e., a p-type SiC semiconductor) are employed, an ohmic electrode having an ohmic junction can be formed by depositing on the p-type semiconductor a metallic compound having a work function higher than that of the p-type semiconductor. However, such a metallic compound has not yet been known. Therefore, in addition to Al, Ti/Al (the expression xe2x80x9cTi/Alxe2x80x9d refers to an electrode formed by depositing Ti and Al successively on the surface of a semiconductor, and the same convention shall apply hereinafter), Al/Ti, or Al/Ni are employed, which utilize diffusion of Al serving as a p-type dopant. Also, a metal silicide which reacts with Si contained in a substrate, such as PtSiX (platinum silicide) or TaSiX (tantalum silicide), is employed. However, a metallic compound containing Al cannot be used reliably at high temperature, since Al has a melting point as low as 660xc2x0 C.
When a metal silicide is employed to form an electrode, in general, the metal silicide is reacted with SiC through, for example, heat treatment, to thereby form an ohmic electrode. However, in this case, Si on the surface of an SiC layer is absorbed into the metal silicide, resulting in generation of a Si depletion layer (i.e., a C segregation layer) on the surface of the SiC layer. Meanwhile, the metal silicide has a portion which reacts with Si and a portion which does not react with Si, and therefore the thickness of the ohmic electrode may vary from portion to portion. Similar problems are expected to arise when a silicide-formable single metal element is employed to form an electrode.
In view of the foregoing, an object of the present invention is to provide an electrode for a silicon carbide semiconductor containing a thermally stable ohmic electrode. When the ohmic electrode is formed, reaction between a metal and a silicon carbide semiconductor (SiC semiconductor); i.e., silicification of the metal, is reduced by causing a silicon layer (Si layer) of low resistance to be present between the metal and the SiC semiconductor. Another object of the present invention is to provide a silicon carbide semiconductor element containing the electrode for a silicon carbide semiconductor, as well as a method for producing the electrode and a method for producing the element.
The electrode for a silicon carbide semiconductor of the present invention has a structure including a metal silicide layer, and a p-type Si layer provided between a p-type SiC semiconductor and the metal silicide layer, such that depletion of Si on the surface layer of the SiC semiconductor is prevented during silicification of the silicide layer. When the Si layer is formed from Si having a carrier concentration equal to or higher than that of SiC, contact resistance between the SiC semiconductor and the Si layer is reduced, to thereby improve properties of an ohmic electrode. The aforementioned p-type SiC semiconductor may be a p-type SiC semiconductor formed by doping an n-type SiC semiconductor with a dopant.
SiC employed in the present invention may be any of various SiC polytypes such as 3C-SiC, 4H-SiC, and 6H-SiC. Preferably, the Si layer is formed from Si having the same conduction-type as SiC, and, as described above, the SiC layer is formed from Si having a carrier concentration equal to or higher than that of the SiC to be employed.
No particular limitation is imposed on the method for depositing Si, and Si may be deposited using a conventional technique such as electron beam deposition, chemical vapor deposition, sputtering, or laser ablation. However, Si is preferably deposited by laser ablation. Deposition of a metal silicide is performed by laser ablation, to thereby form a metal silicide layer. In order to prevent non-uniformity in composition of the metal silicide layer and to maintain excellent ohmic properties, preferably, laser irradiation and/or heat treatment is performed after depositing the metal silicide by laser ablation, to thereby improve ohmic properties and enhance adhesion between a SiC semiconductor, a Si layer, and the metal silicide layer.
When an electrode having a relatively large thickness (e.g., 100 nm or more) is to be formed, preferably, laser irradiation and/or heat treatment is performed after depositing a portion of a metal silicide by laser ablation, to thereby improve ohmic properties and enhance adhesion between a SiC semiconductor, a Si layer, and the metal silicide layer, and then the remaining metal silicide is deposited to thereby form an electrode of predetermined thickness. Through this procedure, ohmic properties can be reliably obtained, and adhesion between the SiC semiconductor and the electrode can be sufficiently enhanced.
Laser irradiation or heat treatment may be performed to improve ohmic properties and adhesion. However, laser irradiation is preferred, so as to reduce diffusion of the material of the electrode into the SiC semiconductor, which occurs in relation to improvement of ohmic properties. When laser irradiation is performed by use of, for example, a KrF excimer laser (xcex: 248 nm, xcfx84p: 20 ns) (energy density: 1 to 2 J/cm2, 50 to 200 pulses), ohmic properties can be sufficiently improved, and adhesion is improved as well. When heat treatment is performed at 800 to 1,200xc2x0 C. for about one minute to one hour, ohmic properties as well as adhesion can also be improved.
Examples of the metal silicide employed for forming a metal silicide layer include suicides of at least one metal element selected from among platinum group elements, Group IVa elements, Group Va elements, Group VIa elements, and Group VIII elements. Of these metal elements, a platinum group element, particularly Pt, is widely employed. In addition to Pt, Ta, Ni, W, etc., can also be employed.
No particular limitation is imposed on the thickness of an Si layer or the thickness of a metal silicide layer prior to improvement of ohmic properties. However, when laser irradiation is performed for improving ohmic properties, the thicknesses of the Si layer and the metal silicide layer must be regulated such that the laser beam can reach the surface of the SiC semiconductor layer. Therefore, the overall thickness of the Si layer and the metal silicide layer prior to laser irradiation is preferably 1 to 100 nm, more preferably 5 to 70 nm, much more preferably 10 to 40 nm. When the overall thickness is less than 1 nm, irradiation of a laser beam may cause ablation of the Si layer and the metal silicide layer, resulting in loss of deposited Si and metal silicide. In contrast, when the overall thickness exceeds 100 nm, the laser beam may fail to reach the surface of the SiC semiconductor, resulting in insufficient improvement of ohmic properties. No particular limitation is imposed on the thickness ratio of the Si layer and the metal silicide layer, but preferably, the Si layer is thinner than the metal silicide layer. Moreover, no limitation is imposed on the overall thickness of an electrode formed through further deposition of the metallic silicide after improvement of ohmic properties.